Flow Lines is a free connect-the-dots logic puzzle you play in the browser: drag a pipe from each colored dot to its twin, without letting the pipes cross. Levels start at 5x5 with four or five pairs and grow to 9x9 with up to nine colors, and every board is generated fresh, so you never replay the same grid twice. It suits anyone who wants a quiet, no-timer thinking puzzle — nothing to download, no account, and your best score is kept on your own device.
How to Play Flow Lines: The Three Rules and the Two Controls Nobody Explains
The rules are short. First: every colored dot has exactly one twin somewhere on the board, and your job is to join the pair with a single unbroken pipe. Second: pipes move only up, down, left and right — never diagonally. Third: two pipes can never occupy the same cell. Connect all the pairs and the level is cleared, and the counter above the board tells you where you stand: "Flows: 3/5" means three pairs are joined and two colors are still loose.
The first control most people miss is the retract. You do not have to restart when a pipe goes wrong — drag backwards along your own pipe and it shortens behind your finger, cell by cell, so you can pull it back three squares and set off in a new direction. You can also press any cell in the middle of an existing pipe and it will trim to that point and continue from there. If you want a color gone entirely, press its dot: that resets that flow to the dot alone and leaves every other color untouched. Restart level, the button under the board, wipes all colors at once — use it only when the whole layout is wrong.
The second control is how the pipe follows your pointer. It walks cell by cell along whichever row or column you share with the cursor, so a fast straight swipe across four squares registers correctly. Diagonal movement is ignored outright — cut across the board at an angle and the pipe simply stops and waits. Move in L shapes: across, then down. Two things to know about collisions. Dragging your pipe over another color's line cuts that line back at the point where you crossed it, which is fast but destructive. Dragging into another color's dot is refused and counts as a wrong move against your score. Lines are cheap to cross by accident; dots are not.
Do You Have to Fill the Whole Board? Perfect Clears vs. Just Connecting the Dots
No. In Flow Lines the level clears the moment the last pair is connected, even if a corner of the grid is still bare. That is deliberately looser than Flow Free, where the board must be completely covered before a level counts as done. So a level you thought was impossible often is not — you may have been trying to fill space that never needed filling.
Full coverage is still worth chasing, because it pays. The "Pipe: %" chip next to the flow counter tracks what share of the grid your pipes occupy, and hitting 100% triggers the PERFECT banner plus a bonus close to the value of clearing the level in the first place. On a 5x5 the clear is worth 70 points and the perfect bonus adds 60; on a 9x9 the clear is worth 94 and perfect adds 76. Finish with gaps and you get a trickle instead — a quarter of your coverage percentage, so around 20 points at best. Across a ten-level run, playing for perfect clears roughly doubles the final score.
There is a structural reason perfect clears are always available: each level is generated by carving the entire grid into paths and then hiding everything except the two ends of each path. A full-coverage solution therefore exists on every board, without exception. When you clear a level at 85% pipe, you did not find a shortcut the generator missed — you found a shorter route, and the empty cells are what it cost you.
The Corner Rule: Why the Edges Solve Themselves First
Look at any corner cell. It has exactly two neighbors, not four. That single fact is the strongest deduction in the game. If a corner holds a dot, that flow's first move is forced into one of only two directions — rule one out and the pipe's opening is decided for you. If a corner is empty and you are chasing full coverage, some pipe must pass through it, and any pipe that does enters through one neighbor and leaves through the other. A corner has no other possible shape.
Edge cells work the same way, one step weaker: three neighbors instead of four, so a pipe running along a wall has fewer options than one in the open middle. This is why the practical order of play is outside-in. Solve the corners, run the border, and by the time you reach the center you are working with far fewer free cells and the middle collapses quickly. Starting in the middle is the most common way to waste ten minutes on an easy board.
One more deduction pays for itself constantly: a pipe should never run alongside itself. If two cells of the same color sit next to each other but are not consecutive along the pipe, you drew a detour — the pipe could have gone straight across there and saved cells. On a perfect-clear attempt that detour is exactly where your missing coverage went, because the squares it wasted had to be stolen from another color. Spot the self-hug, straighten it, and the freed cells usually let the color that was one square short finally reach its dot.
Stuck on a Level? The Five-Step Unsticking Routine
Step one: count what is actually missing. Read the flow counter, not the board. If it says 6/7, exactly one pair is unfinished and every other pipe is potentially fine. Step two: clear only the guilty color by pressing its dot. Most stuck boards are one greedy pipe that took a scenic route through territory another color needed, and wiping all seven flows to fix one is a habit worth breaking.
Step three: hunt for orphan cells. Scan for any empty square whose free neighbors number one or zero. A free cell with a single free neighbor can only ever be the tip of a path, so if it is not sitting next to an unconnected dot, some pipe has walled it off and you have found your bug. A free cell with zero free neighbors is hard proof that the current layout cannot reach a perfect clear. Step four: redraw the trapped region's owner along the wall instead of through the middle, using the corner rule — hug the border and the interior stops fragmenting.
Step five, if the board still resists: clear it and take the ordinary win. Connect every pair by the shortest route available, accept the coverage penalty and move on. Flow Lines has no per-level timer and no lives, so a level cannot be failed, only left imperfect — and because boards are generated rather than fixed, the next one is a genuinely new problem, not a wall you must break through to progress. The points from a scrappy clear still count toward your best.
Flow Lines, Flow Free, or Numberlink? What This Puzzle Is Actually Called
The puzzle has one classical name and a dozen commercial ones. In the puzzle world it is Numberlink, published by the Japanese company Nikoli — the same publisher that popularized Sudoku — where the pairs are matching numbers rather than colored dots and the solver joins them under exactly the rules above. Its ancestor is older still: Sam Loyd's "Puzzle of the Missing Link" from the 1890s ran on the same join-the-pairs-without-crossing idea.
Flow Free is the 2012 mobile game by Big Duck Games that turned the format into a household puzzle, and it is why most people search for the colored-dot version rather than the numbered one. Flow Lines is the same puzzle family with the same three rules. The meaningful difference is the win condition: Flow Free demands total board coverage before a level completes, while Flow Lines completes on connecting all pairs and treats full coverage as the PERFECT bonus. If you have played Flow Free, your instincts transfer intact — you just have more slack.
Anything sold as connect the dots with lines, pipes, link puzzle or Arukone is the same underlying game. It is worth knowing the puzzle is genuinely hard in the mathematical sense, not merely fiddly: deciding whether an arbitrary Numberlink board can be solved is NP-complete, which is the formal reason a 9x9 grid can defeat you while looking almost empty. Humans get through them on local deductions — corners, orphan cells, no self-hugging — rather than by searching every route.
From 5x5 to 9x9: How the Difficulty Curve Works and Who Each Size Suits
The ramp is fixed and gentle. Levels 1 and 2 are 5x5, levels 3 and 4 are 6x6, levels 5 and 6 are 7x7, levels 7 through 10 are 8x8, and from level 11 onward every board is 9x9. Each board carries at least four flows and never more than nine colors, and no single pipe may eat more than a quarter of the grid — six cells on a 5x5, twenty on a 9x9. That cap is what stops a level degenerating into one enormous snake plus three stubs.
For a young child or a first-timer, the 5x5 and 6x6 openers are the right size: four to six colors, short pipes, and the corner rule visible at a glance because there are only 25 cells to scan. No reading is required, there is no clock, and there is no way to lose — which makes the early levels genuinely usable for kids who cannot yet handle a puzzle that punishes mistakes. Most players clear the first four levels in a couple of minutes each.
The jump people actually feel is at 8x8, around level 7. Below that the grid is small enough that a wrong pipe is obvious immediately. At 8x8 and 9x9 you are routing seven to nine colors through 64 to 81 cells, and a bad decision in the top-left will not reveal itself until the bottom-right runs out of room three minutes later. That is where playing outside-in stops being a nice tip and becomes the difference between a perfect clear and a scrappy one. Since level 11 onward stays at 9x9 indefinitely, the ceiling on a run is your patience, not the game's.
Questions people ask
Do I have to fill every square to finish a level?
No. The level clears as soon as the last pair is connected, even with empty cells left over. Covering 100% of the grid triggers the PERFECT banner and a large bonus — on a 9x9 that is 76 points on top of the 94-point clear — but it is optional. Every board is built from a full-coverage solution, so a perfect clear is always available if you want it.
How do I undo a line without restarting the whole level?
Drag backwards along your own pipe and it retracts cell by cell. You can also press any cell partway along a pipe to trim it to that point and redraw from there, or press a color's dot to reset just that flow. Restart level clears every color at once and is rarely what you actually want.
Can lines cross each other?
Never — one cell holds one pipe. If you drag over another color's line, that line is cut back at the crossing point and you take the cell, which is useful when you meant it and painful when you did not. Dragging into another color's dot is blocked outright and is recorded as a wrong move against your score.
Is Flow Lines the same game as Flow Free?
Same puzzle, one rule difference. Both are versions of the classic Numberlink puzzle: connect matching pairs, no crossings, no diagonals. Flow Free requires the whole board to be covered before a level counts as complete; Flow Lines completes on connecting all pairs and pays a bonus for full coverage instead.
Does my progress save, and do I need an account?
No account, no signup, no download — it runs in the browser. Your best score is stored locally on the device you played on, so it is waiting when you come back in the same browser, but it does not follow you to another device, and there are no online leaderboards or multiplayer.
How to play Flow Lines
- Drag between two dots of the same colour to connect them.
- Lines cannot cross — drawing over a line cuts it.
- Connect every pair to win. Cover the whole board for a PERFECT bonus!
Flow Lines is free to play online at Trivio — no download, no signup, works on desktop and mobile. Your best score is saved in your browser.